In recent years, the number of people claiming mental aberrations has noticeably increased, reflecting a stressful society. In particular, the number of people suffering from a depressed mood or depressed state continues to increase as a result of workplace environmental changes as represented by technostress, increases in social stress, physical and mental overwork, disturbed rhythm of living, and the like. As a result of constant anxiety, tension, fretfulness, conflict, and the like, the symptoms of a depressed mood and depressed state are accompanied by affective disorder, in which people inappropriately express delight, anger, sorrow and pleasure; when left untreated, the disorder will develop into depression. It has been reported that about 10 to 15% of patients with depression commit suicide or attempt to do so, making depression an object of public concern.
Hitherto, drugs such as tricyclic antidepressants and monoamine oxidase inhibitors have been used in the treatment of depression. However, these drugs are likely to cause harmful side effects involving the autonomic nervous system and circulatory system; therefore, the long-term administration thereof, the administration in elderly people and the preventive administration thereof have remained difficult. Further, as mentioned above, improvements in depressed mood or depressed state at an early stage is important to prevent depression. However, because people may temporarily experience a depressed mood or depressed state due to environmental changes or when confronting various situations, the depressed mood or depressed state is generally not recognized as a disease. Therefore, preventive treatment therefor is unlikely to be offered until abnormal behavior is recognized. For this reason, an antidepressant that causes fewer side effects, and that enables long-term administration, administration in elderly people and preventive administration, is in demand.
It is known that 10-hydroxydecenoic acid, a compound similar to the carboxylic acid provided by the present invention, exhibits a variety of physiological activities. For example, Patent Document 1 discloses that 10-hydroxydecenoic acid exhibits aldose reductase inhibitory activities, which inhibit, in glucose metabolism, the conversion of glucose to sorbitol, and can thereby prevent or treat diabetes. Further, Patent Document 2 discloses that 10-hydroxydecenoic acid has Jin-Ye effects, i.e., effects of promoting secretion of water from the inside to the outside the body, thus exerting a variety of effects such as beautiful skin effect, hair-growth effect, digestive-juice secretion stimulatory effect, laxative effect, diuretic effect, and the like. However, the antidepressant effects in 10-hydroxydecenoic acid are unknown. Further, the experiments performed by the present inventors confirmed that 10-hydroxydecenoic acid exhibits no antidepressant effects (see Test Example 1 described hereunder).    Patent Document 1: Japanese Unexamined Patent Publication No. H7-69879    Patent Document 2: Japanese Unexamined Patent Publication No. H10-114652